1. Field of the Invention
This invention relates to methods of producing NF.sub.4 BF.sub.4 and its precursor NF.sub.4 SbF.sub.6.
2. Description of the Prior Art
Solid propellant gas generators, such as that described by D. Pilipovich in U.S. Pat. No. 3,963,542, are of great importance for chemical HF-DF lasers. These gas generators are based on NF.sub.4.sup.+ salts. Due to its high NF.sub.3 and F.sub.2 content, NF.sub.4 BF.sub.4 is one of the materials most preferred in making these gas generators. However, there has not previously been an economically feasible method for producing NF.sub.4 BF.sub.4 in a state of sufficient purity to permit its practical application.
Several methods have previously been reported for the synthesis of NF.sub.4 BF.sub.4. This salt has been prepared either directly from NF.sub.3, F.sub.2, and BF.sub.3 using glow discharge, bremsstrahlung or ultraviolet radiation, or indirectly from NF.sub.4 SbF.sub.6 using a metathetical process. Of these, only the metathetical process is amenable to the larger scale production of NF.sub.4 BF.sub.4 utilizing existing technology. The original metathetical NF.sub.4 BF.sub.4 process involved the following steps: EQU CsF + HF .sup.HF CsHF.sub.2 EQU nf.sub.4 sbF.sub.6 + CsHF.sub.2 .sup.HF CsSbF.sub.6 .dwnarw. + NF.sub.4 HF.sub.2 EQU nf.sub.4 hf.sub.2 + bf.sub.3 .sup.hf nf.sub.4 bf.sub.4 + hf
since the crude product, thus obtained, contained much CsSbF.sub.6, its NF.sub.4 BF.sub.4 content had to be increased by extraction with BrF.sub.5. The use of BrF.sub.5 resulted in the following side reaction: EQU NF.sub.4 BF.sub.4 + CsSbF.sub.6 .sup.BrF.sbsp.5 CsBF.sub.4 .dwnarw. + NF.sub.4 SbF.sub.6
the composition of the final product was reported to be: 91.5% NF.sub.4 BF.sub.4 and 8.5% NF.sub.4 SbF.sub.6. In addition to the low purity of the product and the requirement of BrF.sub.5 as a recrystallization solvent, this process suffers from the following disadvantage. Highly concentrated solutions of NF.sub.4 HF.sub.2 in HF are unstable decomposing to NF.sub.3, F.sub.2, and HF. This can cause a pressure build up in the metathesis apparatus which in turn can render filtration steps more difficult.
This process was somewhat improved upon by substituting CsF by AgF. This modification eliminated the BrF.sub.5 extraction step and resulted in a product of the composition (mol %): NF.sub.4 BF.sub.4 (89), NF.sub.4 Sb.sub.2 F.sub.11 (7.9), AgBF.sub.4 (3.1). However, the process still involved the handling of concentrated NF.sub.4 HF.sub.2 solutions and consisted of a rather large number of steps. Furthermore, the cost of silver salts is rather high and, therefore, requires their recycling. This process can be described by the following steps: EQU AgF + HF .sup.in HF AgHF.sub.2 (filtration required to remove impurities) EQU 2 AgHF.sub.2 + NF.sub.4 SbF.sub.6.SbF.sub.5 .sup.in HF 2 AgSbF.sub.6 .dwnarw. + NF.sub.4 HF.sub.2 EQU nf.sub.4 hf.sub.2 + bf.sub.3 .sup.in HF NF.sub.4 BF.sub.4 + HF EQU agHF.sub.2 + BF.sub.3 .sup.in HF AgBF.sub.4 .dwnarw. + HF EQU agSbF.sub.6 .sup..DELTA.T AgF + SbF.sub.5
since these metathetical processes use NF.sub.4 SbF.sub.6 as a precursor, a simple production method for this compound is also desirable. Two methods have previously been reported for the synthesis of NF.sub.4 SbF.sub.6.xSbF.sub.5 involving the use of either high pressure and temperature or uv-irradiation. Of these, the thermal method is more convenient for larger scale production. According to Tolberg et al. the most favorable reaction conditions are: ##EQU1##
The resulting product contained an appreciable amount of Monel salts and was removed from the reactor by cutting it open with a hacksaw and scraping out the hard clinkered product. Based on recent work done in our laboratory, temperatures (250.degree.-260.degree.), higher than those reported by Tolberg, are required for the vacuum pyrolysis of NF.sub.4 SbF.sub.6.xSbF.sub.5 to NF.sub.4 SbF.sub.6 within a reasonable time period.